Classic cemented carbide, i.e., based upon tungsten carbide (WC) and with cobalt (Co) as binder phase has in the last few years met with increased competition from titanium based hard materials, usually named cermets. In the beginning these titanium based alloys were used only for high speed finishing because of their extraordinary wear resistance at high cutting temperatures. This depended essentially upon the good chemical stability of these titanium based alloys. The toughness behaviour and resistance to plastic deformation were not satisfactory, however, and therefore the area of application was relatively limited.
The development has, however, proceeded and the range of application for sintered titanium based hard materials has been considerably enlarged. The toughness behaviour and the resistance to plastic deformation have been considerably improved. This has been done, however, by partly sacrificing the wear resistance.
An important development of titanium based hard alloys is substitution of carbon by nitrogen in the hard constituents. This decreases, i.e., the grain size of the hard constituents in the sintered alloy which, i.e., leads to the possibility of increasing the toughness at unchanged wear resistance. These alloys are usually considerably more fine grained than normal cemented carbide, i.e., WC-Co-based hard alloy. Nitrides are also generally more chemically stable than carbides and these result in lower tendencies to sticking of work piece material or wear by solution of the tool, so called diffusional wear.
In the binder phase, the metals of the iron group, i.e., Fe, Ni and/or Co, are used. In the beginning only Ni was used, but nowadays both Co and Ni are often found in the binder phase of modern alloys.
Besides Ti, the other metals of the groups IVa, Va and VIa, i.e., Zr, Hf, V, Nb, Ta, Cr, Mo and/or W, are normally used as hard constituent formers. There are also other metals used, for example Al, which sometimes are said to harden the binder phase and sometimes improve the wetting between hard constituents and binder phase, i.e., facilitate the sintering.
Most papers, patent publications, etc. relating to sintered carbonitride alloys deal with the hard constituents as a homogeneous phase independent of how many alloying components are involved. This is natural because normally only one type of reflexes is obtained from hard constituents at X-ray diffraction analyses of such alloys. In order for deeper understanding of the often very complex sintered titanium-based carbonitride alloys it is necessary, however, to penetrate the structure more in detail.
It is a general opinion that alloys of this type are always in equilibrium. There are, however, about as many small local equilibriums as the number of hard constituent grains in the alloy. It is evident by way of a more careful examination that the hard constituent grains most often are duplex, usually still more complicated, in the shape of a core and at least one surrounding rim having a different composition. The surrounding rims have within themselves no constant compositions but often contain various gradients at which, for example, a metal content can decrease towards the center, which is compensated for by another metal content which decreases towards the surface. Also, the relative contents of the interstitial elements carbon and nitrogen vary more or less continuously from the center of the hard constituent grains and out to the surface in contact with the binder phase.
U.S. Pat. No. 3,971,656 discloses the preparation of a duplex hard constituent in which the core has a high content of titanium and nitrogen and the surrounding rim has a lower content of these two elements which is compensated for by higher amounts of group VIa-metals, i.e., principally molybdenum and tungsten, and of a higher content of carbon. The higher contents of Mo, W and C have, i.e., the advantage that the wetting to the binder phase is improved, i.e., the sintering is facilitated.
In Swedish Patent Application No. 8604971-5, it is shown how the resistance to plastic deformation can be considerably improved by the carbide phase of the alloy having a duplex structure in which the core has a high content of titanium and tantalum but a low content of nitrogen. The surrounding rim has a higher amount of group VIa-atoms, i.e., molybdenum and tungsten, and a higher nitrogen content than the core, i.e., the distribution of nitrogen is contrary to that of U.S. Pat. No. 3,971,656. In comparison with sintered carbonitride alloys having the same macroscopic compositions but prepared from elementary raw materials (which caused structures of the type described above), a considerably better resistance to plastic deformation was obtained with materials containing duplex carbonitride having a low nitrogen content in the core according to the invention being referred to.
U.S. Pat. No. 4,778,521 relates to carbonitrides with a core containing high amounts of Ti, C and N, an intermediary rim having high amounts of W and C and an outer rim containing Ti, W, C and N in contents between those in the core and those in the intermediary rim, respectively.
Another variation of the same subject is shown in Japanese Patent Application No. 63-216,941 in which the core consists of (Ti, Ta/Nb) (C,N) and the rim of (Ti, Ta/Nb, W/Mo) (C,N). The raw material is the carbonitride of the core and the process is the same as in the previously mentioned patent, i.e., the raw materials with W and Mo are dissolved and are present in the rim which grows on remaining hard constituent grains during the sintering. Also, this type of carbonitride gives an improved toughness at unchanged wear resistance.
It is common in all of the above-mentioned patents and patent applications that they only relate to one type of carbonitride in each sintered alloy and that they have lower contents of group VIa-metals in the core than in the rim/rims.
In German DE 38 06 602 Al is described how the hot strength properties can be improved by giving a raw material in the form of complex carbide and/or nitride a diffusion impeding barrier layer in the beginning of the sintering process, i.e., when the binder phase starts melting, by means of an aluminum containing complex carbide and/or nitride in the raw materials. This is an example of how it is possible by means of so-called "amalgam metallurgy" to isolate cores which otherwise would have been dissolved to some extent. The improved properties are only related to the amount of added Ti.sub.2 AlN.